Nicotine prevents stress-induced enhancement of long-term depression in hippocampal area CA1: electrophysiological and molecular studies

Nicotine inhalant via E-cigarette facilitates sensorimotor function recovery by upregulating neuronal BDNF-TrkB signalling in traumatic brain injury

BACKGROUND AND PURPOSE

Traumatic brain injury (TBI) causes lifelong physical and psychological dysfunction in affected individuals. The current study investigated the effects of chronic nicotine exposure via E-cigarettes (E-cig) (vaping) on TBI-associated behavioural and biochemical changes.

EXPERIMENTAL APPROACH

Adult C57/BL6J male mice were subjected to controlled cortical impact (CCI) followed by daily exposure to E-cig vapour for 6 weeks. Sensorimotor functions, locomotion, and sociability were subsequently evaluated by nesting, open field, and social approach tests, respectively. Immunoblots were conducted to examine the expression of mature brain-derived neurotrophic factor (mBDNF) and associated downstream proteins (p-Erk, p-Akt). Histological analyses were performed to evaluate neuronal survival and neuroinflammation.

KEY RESULTS

Post-injury chronic nicotine exposure significantly improved nesting performance in CCI mice. Histological analysis revealed increased survival of cortical neurons in the perilesion cortex with chronic nicotine exposure. Immunoblots revealed that chronic nicotine exposure significantly up-regulated mBDNF, p-Erk and p-Akt expression in the perilesion cortex of CCI mice. Immunofluorescence microscopy indicated that elevated mBDNF and p-Akt expression were mainly localized within cortical neurons. Immunolabelling of Iba1 demonstrated that chronic nicotine exposure attenuated microglia-mediated neuroinflammation.

CONCLUSIONS AND IMPLICATIONS

Post-injury chronic nicotine exposure via vaping facilitates recovery of sensorimotor function by upregulating neuroprotective mBDNF/TrkB/Akt/Erk signalling. These findings suggest potential neuroprotective properties of nicotine despite its highly addictive nature. Thus, understanding the multifaceted effects of chronic nicotine exposure on TBI-associated symptoms is crucial for paving the way for informed and properly managed therapeutic interventions.

Synaptic Plasticity and Cognitive Ability in Experimental Adult-Onset Hypothyroidism

Adult-onset hypothyroidism impairs normal brain function. Research on animal models of hypothyroidism has revealed critical information on how deficiency of thyroid hormones impacts the electrophysiological and molecular functions of the brain, which leads to the well known cognitive impairment in untreated hypothyroid patients. Currently, such information can only be obtained from experiments on animal models of hypothyroidism. This review summarizes important research findings that pertain to understanding the clinical cognitive consequences of hypothyroidism, which will provide a better guiding path for therapy of hypothyroidism. SIGNIFICANCE STATEMENT: Cognitive impairment occurs during adult-onset hypothyroidism in both humans and animal models. Findings from animal studies validate clinical findings showing impaired long-term potentiation, decreased CaMKII, and increased calcineurin. Such findings can only be gleaned from animal experiments to show how hypothyroidism produces clinical symptoms.

5HT1AR-FGFR1 Heteroreceptor Complexes Differently Modulate GIRK Currents in the Dorsal Hippocampus and the Dorsal Raphe Serotonin Nucleus of Control Rats and of a Genetic Rat Model of Depression

The midbrain raphe serotonin (5HT) neurons provide the main ascending serotonergic projection to the forebrain, including hippocampus, which has a role in the pathophysiology of depressive disorder. Serotonin 5HT1A receptor (R) activation at the soma-dendritic level of serotonergic raphe neurons and glutamatergic hippocampal pyramidal neurons leads to a decrease in neuronal firing by activation of G protein-coupled inwardly-rectifying potassium (GIRK) channels. In this raphe-hippocampal serotonin neuron system, the existence of 5HT1AR-FGFR1 heteroreceptor complexes has been proven, but the functional receptor-receptor interactions in the heterocomplexes have only been investigated in CA1 pyramidal neurons of control Sprague Dawley (SD) rats. In the current study, considering the impact of the receptor interplay in developing new antidepressant drugs, the effects of 5HT1AR-FGFR1 complex activation were investigated in hippocampal pyramidal neurons and in midbrain dorsal raphe serotonergic neurons of SD rats and of a genetic rat model of depression (the Flinders Sensitive Line (FSL) rats of SD origin) using an electrophysiological approach. The results showed that in the raphe-hippocampal 5HT system of SD rats, 5HT1AR-FGFR1 heteroreceptor activation by specific agonists reduced the ability of the 5HT1AR protomer to open the GIRK channels through the allosteric inhibitory interplay produced by the activation of the FGFR1 protomer, leading to increased neuronal firing. On the contrary, in FSL rats, FGFR1 agonist-induced inhibitory allosteric action at the 5HT1AR protomer was not able to induce this effect on GIRK channels, except in CA2 neurons where we demonstrated that the functional receptor-receptor interaction is needed for producing the effect on GIRK. In keeping with this evidence, hippocampal plasticity, evaluated as long-term potentiation induction ability in the CA1 field, was impaired by 5HT1AR activation both in SD and in FSL rats, which did not develop after combined 5HT1AR-FGFR1 heterocomplex activation in SD rats. It is therefore proposed that in the genetic FSL model of depression, there is a significant reduction in the allosteric inhibition exerted by the FGFR1 protomer on the 5HT1A protomer-mediated opening of the GIRK channels in the 5HT1AR-FGFR1 heterocomplex located in the raphe-hippocampal serotonin system. This may result in an enhanced inhibition of the dorsal raphe 5HT nerve cell and glutamatergic hippocampal CA1 pyramidal nerve cell firing, which we propose may have a role in depression.

Molecular insights into the benefits of nicotine on memory and cognition (Review)

The health risks of nicotine are well known, but there is some evidence of its beneficial effects on cognitive function. The present review focused on the reported benefits of nicotine in the brain and summarizes the associated underlying mechanisms. Nicotine administration can improve cognitive impairment in Alzheimer's disease (AD), and dyskinesia and memory impairment in Parkinson's disease (PD). In terms of its mechanism of action, nicotine slows the progression of PD by inhibiting Sirtuin 6, a stress‑responsive protein deacetylase, thereby decreasing neuronal apoptosis and improving neuronal survival. In AD, nicotine improves cognitive impairment by enhancing protein kinase B (also referred to as Akt) activity and stimulating phosphoinositide 3‑kinase/Akt signaling, which regulates learning and memory processes. Nicotine may also activate thyroid receptor signaling pathways to improve memory impairment caused by hypothyroidism. In healthy individuals, nicotine improves memory impairment caused by sleep deprivation by enhancing the phosphorylation of calmodulin‑dependent protein kinase II, an essential regulator of cell proliferation and synaptic plasticity. Furthermore, nicotine may improve memory function through its effect on chromatin modification via the inhibition of histone deacetylases, which causes transcriptional changes in memory‑related genes. Finally, nicotine administration has been demonstrated to rescue long‑term potentiation in individuals with sleep deprivation, AD, chronic stress and hypothyroidism, primarily by desensitizing α7 nicotinic acetylcholine receptors. To conclude, nicotine has several cognitive benefits in healthy individuals, as well as in those with cognitive dysfunction associated with various diseases. However, further research is required to shed light on the effect of acute and chronic nicotine treatment on memory function.

Understanding stress: Insights from rodent models

Through incorporating both physical and psychological forms of stressors, a variety of rodent models have provided important insights into the understanding of stress physiology. Rodent models also have provided significant information with regards to the mechanistic basis of the pathophysiology of stress-related disorders such as anxiety disorders, depressive illnesses, cognitive impairment and post-traumatic stress disorder. Additionally, rodent models of stress have served as valuable tools in the area of drug screening and drug development for treatment of stress-induced conditions. Although rodent models do not accurately reproduce the biochemical or physiological parameters of stress response and cannot fully mimic the natural progression of human disorders, yet, animal research has provided answers to many important scientific questions. In this review article, important studies utilizing a variety of stress models are described in terms of their design and apparatus, with specific focus on their capabilities to generate reliable behavioral and biochemical read-out. The review focusses on the utility of rodent models by discussing examples in the literature that offer important mechanistic insights into physiologically relevant questions. The review highlights the utility of rodent models of stress as important tools for advancing the mission of scientific research and inquiry.

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Stressful life events may lead to the onset of severe psychopathologies in humans.

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Rodents may model many features of stress exposure in human populations.

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Induction of stress via pharmacological and psychological manipulations alter rodent behavior.

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Mechanistic rodent studies reveal key molecular targets critical for new therapeutic targets.

Delayed effects of combined stress and Aβ infusion on L-LTP of the dentate gyrus: Prevention by nicotine

Alzheimer's Disease (AD) is a progressive dementia hallmarked by the presence in the brain of extracellular beta-amyloid (Aβ) plaques and intraneuronal fibrillary tangles. Chronic stress is associated with heightened Aβ buildup and acceleration of development of AD, however, stress alone has no significant effect on synaptic plasticity in the dentate gyrus (DG) area. Previously, we have reported that the combination of stress and AD causes more severe inhibition of synaptic plasticity of hippocampal area CA1 than chronic stress or AD alone, and that chronic nicotine treatment prevents this impairment. To investigate the effect of stress and nicotine on synaptic plasticity in the relatively injury-resistant DG area, the present experiments analyzed the effect of chronic stress and the neuroprotective effect of nicotine on LTP in the DG area of a rat model of AD. Wistar rats were chronically stressed and treated with nicotine (1 mg/kg/twice daily; s.c.) for six weeks. Then, at weeks 5-6, AD model was generated by 14-day i.c.v osmotic pump infusion of Aβ peptides (300 pmol/day) into the brains of these rats. Field potential recordings from the DG area of anesthetized rats, revealed that while chronic stress did not accentuate Aβ-induced impairments of E-LTP, it markedly augmented Aβ effect on L-LTP that was only seen 100 min after multiple high frequency stimulation. This delayed action is likely to be due to impairment of process of de novo protein synthesis required for maintenance phase of L-LTP. Chronic nicotine treatment prevented stress-enhanced suppression of synaptic plasticity.

Nicotine-prevented learning and memory impairment in REM sleep-deprived rat is modulated by DREAM protein in the hippocampus

INTRODUCTION

REM sleep deprivation is associated with impairment in learning and memory, and nicotine treatment has been shown to attenuate this effect. Recent studies have demonstrated the importance of DREAM protein in learning and memory processes. This study investigates the association of DREAM protein in REM sleep-deprived rats hippocampus upon nicotine treatment.

METHODS

Male Sprague Dawley rats were subjected to normal condition, REM sleep deprivation and control wide platform condition for 72 hr. During this procedure, saline or nicotine (1 mg/kg) was given subcutaneously twice a day. Then, Morris water maze (MWM) test was used to assess learning and memory performance of the rats. The rats were sacrificed and the brain was harvested for immunohistochemistry and Western blot analysis.

RESULTS

MWM test found that REM sleep deprivation significantly impaired learning and memory performance without defect in locomotor function associated with a significant increase in hippocampus DREAM protein expression in CA1, CA2, CA3, and DG regions and the mean relative level of DREAM protein compared to other experimental groups. Treatment with acute nicotine significantly prevented these effects and decreased expression of DREAM protein in all the hippocampus regions but only slightly reduce the mean relative level of DREAM protein.

CONCLUSION

This study suggests that changes in DREAM protein expression in CA1, CA2, CA3, and DG regions of rat's hippocampus and mean relative level of DREAM protein may involve in the mechanism of nicotine treatment-prevented REM sleep deprivation-induced learning and memory impairment in rats.

Caffeine treatment prevents rapid eye movement sleep deprivation-induced impairment of late-phase long-term potentiation in the dentate gyrus

The CA1 and dentate gyrus (DG) are physically and functionally closely related areas of the hippocampus, but they differ in various respects, including their reactions to different insults. The purpose of this study was to determine the protective effects of chronic caffeine treatment on late-phase long-term potentiation (L-LTP) and its signalling cascade in the DG area of the hippocampus of rapid eye movement sleep-deprived rats. Rats were chronically treated with caffeine (300 mg/L drinking water) for 4 weeks, after which they were sleep-deprived for 24 h. L-LTP was induced in in anaesthetized rats, and extracellular field potentials from the DG area were recorded in vivo. The levels of L-LTP-related signalling proteins were assessed by western blot analysis. Sleep deprivation markedly reduced L-LTP magnitude, and basal levels of total cAMP response element-binding protein (CREB), phosphorylated CREB (P-CREB), and calcium/calmodulin kinase IV (CaMKIV). Chronic caffeine treatment prevented the reductions in the basal levels of P-CREB, total CREB and CaMKIV in sleep-deprived rats. Furthermore, caffeine prevented post-L-LTP sleep deprivation-induced downregulation of P-CREB and brain-derived neurotrophic factor in the DG. The current findings show that caffeine treatment prevents acute sleep deprivation-induced deficits in brain function.

The Effect of Waterpipe Tobacco Smoke Exposure on Learning and Memory Functions in the Rat Model

Tobacco smoking is a global health hazard that kills about 5 million people annually. Waterpipe smoking is among the most popular methods of tobacco consumption worldwide. In this study, we investigated whether waterpipe smoking impairs learning and memory in the hippocampus, a question of special concern due to the particular popularity of waterpipe use among youth. Additionally, possible molecular targets for expected learning and memory impairment were determined. In this study, rats were exposed to waterpipe smoke (WTS) by whole body exposure 1 h × 5 days/week, for 1 month, and outcomes were compared to a control group exposed only to fresh air. Outcomes included spatial learning and memory using the radial arm water maze (RAWM) and oxidative stress biomarkers (catalase, glutathione peroxidase (GPx), superoxide dismutase (SOD), glutathione (GSH), oxidized glutathione (GSSG), GSH/GSSG ratio). We found that WTS exposure led to impaired short- and long-term memory. This impairment was accompanied by reduced hippocampal activity of catalase, SOD, GPx, GSH, and GSH/GSSG, elevated GSSG, thus marked changes in oxidative stress biomarkers. In conclusion, there is reason for concern that WTS exposure may impair cognitive ability.

Smoking restores impaired LTD-like plasticity in schizophrenia: a transcranial direct current stimulation study

Impaired neuroplastic responses following noninvasive brain stimulation have been reported repeatedly in schizophrenia patients. These findings have been associated with deficits in GABAergic, glutamatergic, and cholinergic neurotransmission. Although various neurophysiological studies have indicated a relationship between nicotine and neuroplasticity in healthy individuals, the present study is the first investigation into the impact of nicotine on LTD-like plasticity in patients with schizophrenia. Cortical excitability and cortical plasticity were explored in 30 schizophrenia patients (17 smoker, 13 nonsmoker) and 45 healthy controls (13 smoker, 32 nonsmoker) by using single-pulse transcranial magnetic stimulation (TMS) before and following cathodal transcranial direct current stimulation (tDCS) applied to the left primary motor cortex. Our analysis revealed abolished LTD-like plasticity in nonsmoking schizophrenia patients. However, these plasticity deficits were not present in smoking schizophrenia patients. In healthy controls, significant MEP reductions following cathodal tDCS were observed in nonsmoking individuals, but only trend-level reductions in smokers. In smoking schizophrenia patients, the severity of negative symptoms correlated positively with reduced neuroplasticity, whereas nonsmoking patients displayed the opposite effect. Taken together, the data of our study support the notion of an association between chronic smoking and the restitution of impaired LTD-like plasticity in schizophrenia patients. Although replication and further research are needed to better understand this relationship, our findings indicate that nicotine intake might stabilize the impaired inhibition-facilitation balance in the schizophrenic brain through a complex interaction between cortical plasticity, and GABAergic and cholinergic neurotransmission, and might explain the reduced prevalence of negative symptoms in this population.

Brain Physiology and Pathophysiology in Mental Stress

Exposure to various forms of stress is a common daily occurrence in the lives of most individuals, with both positive and negative effects on brain function. The impact of stress is strongly influenced by the type and duration of the stressor. In its acute form, stress may be a necessary adaptive mechanism for survival and with only transient changes within the brain. However, severe and/or prolonged stress causes overactivation and dysregulation of the hypothalamic pituitary adrenal (HPA) axis thus inflicting detrimental changes in the brain structure and function. Therefore, chronic stress is often considered a negative modulator of the cognitive functions including the learning and memory processes. Exposure to long-lasting stress diminishes health and increases vulnerability to mental disorders. In addition, stress exacerbates functional changes associated with various brain disorders including Alzheimer’s disease and Parkinson’s disease. The primary purpose of this paper is to provide an overview for neuroscientists who are seeking a concise account of the effects of stress on learning and memory and associated signal transduction mechanisms. This review discusses chronic mental stress and its detrimental effects on various aspects of brain functions including learning and memory, synaptic plasticity, and cognition-related signaling enabled via key signal transduction molecules.

Role of α7- and α4β2-nAChRs in the neuroprotective effect of nicotine in stress-induced impairment of hippocampus-dependent memory

We have previously shown that nicotine prevents stress-induced memory impairment. In this study, we have investigated the role of α7- and α4β2-nicotinic acetylcholine receptors (nAChRs) in the protective effect of nicotine during chronic stress conditions. Chronic psychosocial stress was induced using a form of rat intruder model. During stress, specific antagonist for either α7-nAChRs [methyllycaconitine (MLA)] or α4β2-nAChRs [dihydro-β-erythroidine (DHβE)] was infused into the hippocampus using a 4-wk osmotic pump at a rate of 82 μg/side.d and 41 μg/side.d, respectively. Three weeks after the start of infusion, all rats were subjected to a series of cognitive tests in the radial arm water maze (RAWM) for six consecutive days or until the animal reached days to criterion (DTC) in the fourth acquisition trial and in all memory tests. DTC is defined as the number of days the animal takes to make no more than one error in three consecutive days. In the short-term memory test, MLA-infused stressed/nicotine-treated rats made similar errors to those of stress and significantly more errors compared to those of stress/nicotine, nicotine or control groups. This finding was supported by the DTC values for the short memory tests. Thus, MLA treatment blocked the neuroprotective effect of nicotine during chronic stress. In contrast, DHβE infusion did not affect the RAWM performance of stress/nicotine animals. These results strongly suggest the involvement of α7-nAChRs, but not α4β2-nAChRs, in the neuroprotective effect of chronic nicotine treatment during chronic stress conditions.

Evaluation of the effect of pentoxifylline on sleep-deprivation induced memory impairment

In this study, we examined the ability of Pentoxifylline (PTX) to prevent sleep deprivation induced memory impairment probably through decreasing oxidative stress. Sleep deprivation was chronically induced 8 h/day for 6 weeks in rats using modified multiple platform model. Concurrently, PTX (100 mg/kg) was administered to animals on daily basis. After 6 weeks of treatment, behavioral studies were conducted to test the spatial learning and memory using the Radial Arm Water Maze. Additionally, the hippocampus was dissected; and levels/activities of antioxidant defense biomarkers glutathione reduced (GSH), glutathione oxidized (GSSG), GSH/GSSG ratio, glutathione peroxidase (GPx), catalase, and superoxide dismutase (SOD), were assessed. The results show that chronic sleep deprivation impaired short- and long-term memories, which was prevented by chronic treatment with PTX. Additionally, PTX normalized sleep deprivation-induced reduction in the hippocampus GSH/GSSG ratio (P < 0.05), and activities of GPx, catalase, and SOD (P < 0.05). In conclusion, chronic sleep deprivation induces memory impairment, and treatment with PTX prevented this impairment probably through normalizing antioxidant mechanisms in the hippocampus.

Chronic stress and Alzheimer's disease-like pathogenesis in a rat model: prevention by nicotine

Environmental factors including chronic stress may play a critical role in the manifestation of Alzheimer's disease (AD).This review summarizes our studies of the aggravation of the impaired cognitive ability and its cellular and molecular correlates by chronic psychosocial stress and prevention by nicotine in an Aβ rat model of AD. We utilized three approaches: learning and memory tests in the radial arm water maze, electrophysiological recordings of the cellular correlates of memory, long-term potentiation (LTP) and long-term depression (LTD), in anesthetized rats, and immunoblot analysis of synaptic plasticity- and cognition-related signaling molecules. The Aβ rat model, representing the sporadic form of established AD, was induced by continuous i.c.v. infusion of a pathogenic dose of Aβ peptides via a 14- day osmotic pump. In this AD model, chronic stress intensified cognitive deficits, accentuated the disruption of signaling molecules levels and produced greater depression of LTP than what was seen with Aβ infusion alone. Chronic treatment with nicotine was highly efficient in preventing the effects of Aβ infusion and the exacerbating impact of chronic stress. Possible mechanisms for the effect of chronic stress are discussed.

Neuroplasticity in cigarette smokers is altered under withdrawal and partially restituted by nicotine exposition

Nicotine improves cognitive functions by modulating neuroplasticity and cortical excitability in nonsmoking subjects. As shown recently, the positive effect of nicotine on cognition might at least partially be caused by a focusing effect of nicotine on neuroplasticity in these subjects. Concordant to this, smokers under nicotine withdrawal show reduced cognitive abilities, which are at least partially restituted by nicotine consumption. We aimed to explore the neurophysiological foundation of these effects by exploring nonfocal and focal plasticity-inducing protocols in human smokers under nicotine withdrawal and exposition. Focal, synapse-specific plasticity was induced by paired associative stimulation (PAS), while nonfocal plasticity was induced by transcranial direct current stimulation (tDCS). Each subject (12) received placebo and nicotine patches combined with one of the stimulation protocols to the primary motor cortex. Corticospinal excitability was monitored by transcranial magnetic stimulation-induced motor-evoked potential amplitudes. In smokers during nicotine withdrawal, facilitatory plasticity induced by tDCS and PAS was abolished, but restituted by nicotine. In contrast, excitability-diminishing plasticity was not affected by nicotine withdrawal. Under nicotine, the inhibitory aftereffects of PAS were delayed and prolonged, while the tDCS-generated excitability reduction was abolished. Thus, absent facilitatory plasticity in smokers during nicotine withdrawal is restituted by nicotine, favoring the deficit-compensating hypothesis of nicotine consumption. These results might shed further light on the proposed mechanism of nicotine on cognition and attention, which might be connected to nicotine addiction and probability of relapse in smokers.

Chronic psychosocial stress enhances long-term depression in a subthreshold amyloid-beta rat model of Alzheimer's disease

In addition to genetic aspects, environmental factors such as stress may also play a critical role in the etiology of the late onset, sporadic Alzheimer's disease (AD). The present study examined the effect of chronic psychosocial stress in a sub-threshold Aβ (subAβ) rat model of AD on long-term depression by two techniques: electrophysiological recordings of synaptic plasticity in anesthetized rats, and immunoblot analysis of memory- and AD-related signaling molecules. Chronic psychosocial stress was induced using a rat intruder model. The subAβ rat model of AD, which was intended to represent outwardly normal individuals with a pre-disposition to AD, was induced by continuous infusion of 160 pmol/day Aβ₁₋₄₂ via a 14-day i.c.v. osmotic pump. Results from electrophysiological recordings showed that long-term depression evoked in stress/subAβ animals was significantly enhanced compared with that in animals exposed to stress or subAβ infusion alone. Molecular analysis of various signaling molecules 1 h after induction of long-term depression revealed an increase in the levels of calcineurin and phosphorylated CaMKII in groups exposed to stress compared with other groups. The levels of the brain-derived neurotrophic factor (BDNF) were significantly decreased in stress/subAβ animals but not in stress or subAβ animals. In addition, the levels of beta-site amyloid precursor protein cleaving enzyme were markedly increased in stress/subAβ. These findings suggest that chronic stress may accelerate the impairment of synaptic plasticity and consequently cognition in individuals 'at-risk' for AD.

The neuroprotective effect of vitamin E on chronic sleep deprivation-induced memory impairment: the role of oxidative stress

Sleep deprivation induces oxidative stress and impairs learning and memory processes. Vitamin E, on the other hand, is a strong antioxidant that has neuroprotective effect on the brain. In this study, we examined the potential protective effect of chronic administration of vitamin E on chronic sleep deprivation-induced cognitive impairment. In addition, possible molecular targets for vitamin E effects on chronic sleep deprivation-induced cognitive impairment were determined. Sleep deprivation was induced in rats using modified multiple platform model. Vitamin E (100mg/kg) was administered to animals by oral gavage. Behavioral study was conducted to test the spatial learning and memory using the radial arm water maze (RAWM). In addition, the hippocampus was dissected out and antioxidant markers including glutathione (GSH), oxidized glutathione (GSSG) and GSH/GSSG, glutathione peroxidase (GPx), catalase, and superoxide dismutase (SOD) were assessed. The results of this project revealed that chronic sleep deprivation impaired both (short- and long-term) memories (P<0.05), while vitamin E treatment prevented such effect. Additionally, vitamin E normalized chronic sleep deprivation-induced reduction in the hippocampus GSH/GSSG ratio, and activity of catalase, SOD, and GPx. In conclusion, sleep deprivation induces memory impairment, and treatment with vitamin E prevented this impairment probably through its antioxidant action in the hippocampus.

The cholinergic system and hippocampal plasticity

Acetylcholine is an essential excitatory neurotransmitter in the central nervous system and undertakes a vital role in cognitive function. Consequently, there is ample evidence to suggest the involvement of both nicotinic and muscarinic acetylcholine receptors in the modulation of synaptic plasticity, which is believed to be the molecular correlate of learning and memory. In the hippocampus in particular, multiple subtypes of both nicotinic and muscarinic receptors are present at presynaptic and postsynaptic loci of both principal neurons and inhibitory interneurons, where they exert profound bi-directional influences on synaptic transmission. Further evidence points to a role for cholinergic activation in the induction and maintenance of synaptic plasticity, and key influences on hippocampal network oscillations. The present review examines these multiple roles of acetylcholine in hippocampal plasticity.

Changes in spatial memory and BDNF expression to concurrent dietary restriction and voluntary exercise

Substantial data suggest that cognitive function can be influenced by many lifestyle activities associated with changes in energy metabolism such as exercise and diet. In the current study, we investigated the combined effects of voluntary exercise (access to running wheels) and dietary restriction (every other day fasting, EODF) on spatial memory formation and on the levels of brain-derived neurotrophic factor (BDNF) in the hippocampus of Wistar male rats. Spatial learning and memory formation was assessed using the radial arm water maze (RAWM) paradigm, while BDNF protein was measured using ELISA test. Voluntary exercise and/or EODF were instituted for 6 weeks. Voluntary exercise alone significantly enhanced short-term, intermediate-term, and long-term memory formation, and increased BDNF protein levels in the hippocampus. EODF enhanced mean running wheel activity by approximately twofold. However, EODF did not modulate the effects of exercise on memory formation and expression of BDNF. In addition, EODF alone had no effect on memory and BDNF protein in the hippocampus. In conclusion, results of this study indicate that exercise enhanced while EODF had neutral effect on both spatial memory formation and hippocampus BDNF levels.

Intensification of long-term memory deficit by chronic stress and prevention by nicotine in a rat model of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by cholinergic dysfunction and deposition of beta-amyloid (Aβ) in regions of the brain associated with learning and memory. The sporadic nature and late onset of most AD cases suggests that aside from biological determinants, environmental factors such as stress may also play a role in the progression of the disease. Behavioral and molecular studies were utilized to evaluate the effects of chronic nicotine treatment in the prevention of impairment of long-term memory. The rat model of AD was induced by i.c.v. osmotic pump infusion of Aβ peptides. Chronic psychosocial stress and chronic nicotine treatment were instituted for 6weeks. Spatial memory testing in the Radial Arm Water Maze revealed that, although stress, by itself, did not affect long-term memory, the combination of chronic stress and Aβ infusion impaired long-term memory significantly more than Aβ peptides infusion alone. Chronic nicotine treatment completely prevented Aβ- and stress/Aβ combination-induced memory impairment. Furthermore, molecular findings in hippocampal CA1 region of stress/Aβ rats indicated marked reduction in the protein levels of phosphorylated cAMP response element binding (p-CREB) and calcium-calmodulin-dependent protein kinase IV (CaMKIV), with significant increases in the levels of brain-derived neurotrophic factor (BDNF). These disturbances in signaling pathways, which may be the underlying mechanisms of impairment of long-term memory in these rats, were totally prevented by chronic nicotine treatment.

Acute nicotine treatment prevents REM sleep deprivation-induced learning and memory impairment in rat

Rapid eye movement (REM) sleep deprivation (SD) is implicated in impairment of spatial learning and memory and hippocampal long-term potentiation (LTP). An increase in nicotine consumption among habitual smokers and initiation of tobacco use by nonsmokers was observed during SD. Although nicotine treatment was reported to attenuate the impairment of learning and memory and LTP associated with several mental disorders, the effect of nicotine on SD-induced learning and memory impairment has not been studied. Modified multiple platform paradigm was used to induce SD for 24 or 48 h during which rats were injected with saline or nicotine (1 mg kg(-1) s.c.) twice a day. In the radial arm water maze (RAWM) task, 24- or 48-h SD significantly impaired learning and short-term memory. In addition, extracellular recordings from CA1 and dentate gyrus (DG) regions of the hippocampus in urethane anesthetized rats showed a significant impairment of LTP after 24- and 48-h SD. Treatment of normal rats with nicotine for 24 or 48 h did not enhance spatial learning and memory or affect magnitude of LTP in the CA1 and DG regions. However, concurrent, acute treatment of rats with nicotine significantly attenuated SD-induced impairment of learning and STM and prevented SD-induced impairment of LTP in the CA1 and DG regions. These results show that acute nicotine treatment prevented the deleterious effect of sleep loss on cognitive abilities and synaptic plasticity.

Association of a polymorphism in the NRXN3 gene with the degree of smoking in schizophrenia: a preliminary study

Whole genome scan studies have recently identified the NRXN1 and NRXN3 genes as potential contributing factors in the risk for nicotine addiction. We have genotyped 15 single nucleotide polymorphisms (SNPs) spanning the NRXN1 and NRXN3 genes in 195 unrelated patients with schizophrenia for whom information about their smoking status and number of cigarettes smoked per day (CPD) was obtained. The NRXN3 marker rs1004212 was significantly associated with quantity of tobacco smoked. Individuals homozygous for the C allele of rs1004212 smoked more cigarettes per day than heterozygous individuals. We found no significant association of markers within the NRXN1 gene with the risk of smoking or the quantity of tobacco smoked. Because of the relatively small sample size, this is a preliminary study. However, this candidate gene study supports the observations of molecular studies implicating the NRXN genes in drug addiction and suggests that variants in the NRXN3 gene could contribute to the degree of nicotine dependence in patients with schizophrenia.

Chronic psychosocial stress exacerbates impairment of cognition and long-term potentiation in beta-amyloid rat model of Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is a degenerative disorder that leads to progressive cognitive decline. Alzheimer's disease develops as a result of over-production and aggregation of beta-amyloid (Abeta) peptides in the brain. The reason for variation in the gravity of symptoms among AD patients is unknown and might result from patient-related factors including lifestyle. Individuals suffering from chronic stress are at an increased risk for developing AD. This study investigated the effect of chronic psychosocial stress in Abeta rat model of AD.

METHODS

Psychosocial stress was induced with a rat intruder model. The rat model of AD was induced by 14-day osmotic pump infusion of a mixture of 300 pmol/day Abeta(1-40)/Abeta(1-42). The effect of chronic stress on the severity of Abeta-induced spatial learning and memory impairment was tested by three approaches: behavioral testing in the radial arm water maze, in vivo electrophysiological recording in anesthetized rat, and immunoblot analysis to determine protein levels of learning- and memory-related molecules.

RESULTS

A marked impairment of learning and memory developed when stress was combined with Abeta, more so than that caused by Abeta alone. Additionally, there was a significantly greater impairment of early-phase long-term potentiation (E-LTP) in chronically stressed/Abeta-treated rats than in either the stressed or Abeta-treated rats. This might be a manifestation of the reduction in protein levels of calcium/calmodulin-dependent protein kinase II (CaMKII) and the abnormal increase in calcineurin levels.

CONCLUSIONS

Chronic stress significantly intensified Abeta-induced deficits of short-term memory and E-LTP by a mechanism involving decreased CaMKII activation along with increased calcineurin levels.

Chronic nicotine restores normal Aβ levels and prevents short-term memory and E-LTP impairment in Aβ rat model of Alzheimer's disease

Alzheimer's disease (AD) is a devastating neurodegenerative disorder characterized by increased deposition of beta-amyloid (Aβ) peptides and progressive cholinergic dysfunction in regions of the brain involved in learning and memory processing. In AD, progressive accumulation of Aβ peptide impairs nicotinic acetylcholine receptor (nAChR) function by an unknown mechanism believed to involve α(7)- and α(4)β(2)-nAChR blockade. The three approaches of the current study evaluated the effects of chronic nicotine treatment in the prevention of Aβ-induced impairment of learning and short-term memory. Rat AD model was induced by 14-day i.c.v. osmotic pump infusion of a 1:1 mixture of 300 pmol/day Aβ(1-40)/Aβ(1-42) or Aβ(40-1) (inactive peptide, control). The effect of nicotine (2 mg/(kg day)) on Aβ-induced spatial learning and memory impairments was assessed by evaluation of performance in the radial arm water maze (RAWM), in vivo electrophysiological recordings of early-phase long-term potentiation (E-LTP) in urethane-anesthetized rats, and immunoblot analysis to determine changes in the levels of beta-site amyloid precursor protein (APP)-cleaving enzyme (BACE), Aβ and memory-related proteins. The results indicate that 6 weeks of nicotine treatment reduced the levels of Aβ(1-40) and BACE1 peptides in hippocampal area CA1 and prevented Aβ-induced impairment of learning and short-term memory. Chronic nicotine also prevented the Aβ-induced inhibition of basal synaptic transmission and LTP in hippocampal area CA1. Furthermore, chronic nicotine treatment prevented the Aβ-induced reduction of α(7)- and α(4)-nAChR. These effects of nicotine may be due, at least in part, to upregulation of brain derived neurotropic factor (BDNF).

Effect of chronic stress or nicotine on hypothyroidism-induced enhancement of LTD: electrophysiological and molecular studies

We have shown recently that either hypothyroidism or chronic psychosocial stress enhances the expression of LTD, which is reversed by chronic nicotine treatment. In this study, we investigated the effect of combining chronic psychosocial stress with hypothyroidism on LTD. We have also investigated the levels of signaling molecules important for LTD in hypothyroid, stressed-hypothyroid and nicotine-treated hypothyroid rats. Following paired pulse stimulation, LTD was evoked in the CA1 region of anesthetized rats. Combining chronic psychosocial stress with hypothyroidism does not further enhance LTD magnitude compared to either alone. Western blot analysis conducted 1 h after induction of LTD, showed that the levels of calcineurin and P-CaMKII were increased in hypothyroid and stressed-hypothyroid groups compared to that of the control group. However, the levels of calcineurin and P-CaMKII after paired pulsed stimulation were not further increased in stressed-hypothyroid group compared to the hypothyroid only group. In addition, these levels were normalized by chronic nicotine treatment. No change was detected in any of the groups in the levels of calmodulin, PKCgamma, and BDNF after paired pulse stimulation. Our results indicate that changes in the levels of calcineurin and P-CaMKII during expression of LTD in the CA1 region may explain the enhanced magnitude of LTD in hypothyroid rats, and its reversal by chronic nicotine treatment.

The role of the hippocampus in mediating emotional responses to nicotine and cannabinoids: a possible neural substrate for functional interactions

The endocannabinoid system is involved in the regulation of behavioural and physiological stress-related responses. Nicotine exerts complex effects on emotional behaviour, and its withdrawal may result in depressive and anxiogenic-like symptoms. Cannabinoid receptor agonists and nicotine induce biphasic effects in diverse tests of unconditioned anxiety, alter adrenocortical activity and affect hippocampus-dependent contextual fear conditioning. Upon exposure to stressful stimuli, central endocannabinoid and cholinergic systems appear to be activated in key limbic areas such as hippocampus and amygdala, which might contribute to adaptive cognitive and emotional strategies to cope with aversive situations. Numerous studies indicate the existence of functional interactions between nicotine and cannabinoids, particularly in relation to anxiety-related processes. An overlapping distribution of CB1 and nicotinic acetylcholine receptors in the hippocampus is observed and the endocannabinoid system exerts a modulatory role over the hippocampal cholinergic system. In this review, we point to the hippocampus as a relevant neural substrate for cannabinoid-nicotine interactions, notably as regards emotional responses. After a general description of the cannabinoid and nicotinic systems, we review their implications in unconditioned anxiety, depressive-like behaviour and fear conditioning. Then we discuss the role of both systems in modulating stress-induced changes at cellular, endocrine and behavioural levels and their possible involvement in hippocampal neurogenesis. Although we mainly focus on animal data, some relevant human studies are also discussed.

Adult-onset hypothyroidism facilitates and enhances LTD: Reversal by chronic nicotine treatment

Chronic nicotine treatment reverses hypothyroidism-induced impairment of hippocampus-dependent spatial memory and long-term potentiation (LTP). We investigated the effect of hypothyroidism on long-term depression (LTD) and possible protection by nicotine. Following paired pulse stimulation, LTD was expressed in hippocampal area CA1 of anesthetized thyroidectomized, euthyroid (sham control), nicotine-treated and nicotine-treated thyroidectomized (hypothyroid) rats. In hypothyroid rats, a significantly higher LTD magnitude was seen compared with that in control rats. A brief train of stimuli (5 pulses at 100 Hz), which did not affect synaptic transmission in control rats, induced a robust LTD in hypothyroid rats suggesting facilitation of LTD expression in these rats. Chronic nicotine treatment (1 mg/kg, 2x day) of hypothyroid rats reversed hypothyroidism-induced enhancement and facilitation of LTD. Western blot analysis of the NMDA receptor subunits in the membranous fractions of hippocampal area CA1 neurons revealed that hypothyroidism reduced NR1 and increased NR2B without affecting NR2A protein levels. These changes in NMDA receptors in hypothyroid rats were reversed by chronic nicotine treatment. Hypothyroidism did not alter BDNF or nicotinic acetylcholine receptor (nAChR) levels. However, nicotine treatment increased protein levels of these molecules in both euthyroid and hypothyroid rats. Our results suggest that alterations in the levels of NMDA receptor subunits may account for the facilitation and enhancement of LTD in hypothyroidism.

The temporal dynamics model of emotional memory processing: a synthesis on the neurobiological basis of stress-induced amnesia, flashbulb and traumatic memories, and the Yerkes-Dodson law

We have reviewed research on the effects of stress on LTP in the hippocampus, amygdala and prefrontal cortex (PFC) and present new findings which provide insight into how the attention and memory-related functions of these structures are influenced by strong emotionality. We have incorporated the stress-LTP findings into our "temporal dynamics" model, which provides a framework for understanding the neurobiological basis of flashbulb and traumatic memories, as well as stress-induced amnesia. An important feature of the model is the idea that endogenous mechanisms of plasticity in the hippocampus and amygdala are rapidly activated for a relatively short period of time by a strong emotional learning experience. Following this activational period, both structures undergo a state in which the induction of new plasticity is suppressed, which facilitates the memory consolidation process. We further propose that with the onset of strong emotionality, the hippocampus rapidly shifts from a "configural/cognitive map" mode to a "flashbulb memory" mode, which underlies the long-lasting, but fragmented, nature of traumatic memories. Finally, we have speculated on the significance of stress-LTP interactions in the context of the Yerkes-Dodson Law, a well-cited, but misunderstood, century-old principle which states that the relationship between arousal and behavioral performance can be linear or curvilinear, depending on the difficulty of the task.

Nicotine reverses adult-onset hypothyroidism-induced impairment of learning and memory: Behavioral and electrophysiological studies

Nicotine alleviates cognitive impairment associated with a variety of health conditions. We examined the effect of chronic nicotine treatment on adult-onset hypothyroidism-induced impairment of learning and memory in rats. Hypothyroidism was induced by surgical removal of thyroid glands (thyroidectomy). One month later, chronic nicotine treatment (1 mg/kg sc, twice/day) was instituted for 4-6 weeks. Test of hippocampus-dependent spatial learning and memory in the radial arm water maze showed that hypothyroidism impaired learning as well as short-term and long-term memory retention. Chronic nicotine treatment reversed the hypothyroidism-induced learning and memory impairment. In normal rats, chronic nicotine treatment had no effect on learning and memory. Extracellular recordings from the CA1 region of anesthetized hypothyroid rats showed severe reduction of both early-phase and late-phase long-term potentiation (LTP) magnitude, which was reversed in nicotine-treated hypothyroid rats. These results show that chronic nicotine treatment prevents hypothyroidism-induced impairment of spatial cognition and LTP.

Chronic but not acute nicotine treatment reverses stress-induced impairment of LTP in anesthetized rats

Stress impairs long-term potentiation (LTP) and is a major cause for starting or increasing tobacco smoking. We have previously shown that chronic concurrent nicotine treatment prevents stress-induced LTP impairment. Nicotine reduces stress-induced impairment of LTP, probably, through activation of nicotinic acetylcholine receptors in the hippocampus. Herein, we investigated the effects of acute and chronic nicotine treatments on the chronic-stress-induced impairment of LTP in area CA1 of the hippocampus of urethane-anesthetized rats. Extracellular in vivo recording from the hippocampal area CA1 showed that pre-treatment with nicotine (1 mg/kg; sc twice/day for 2 weeks prior to stress) protected LTP from the inhibitory effect of subsequent chronic psychosocial stress (4 additional weeks concurrently with nicotine). In another series of experiments, 2 weeks of psychosocial stress was followed by 4 weeks of nicotine treatment concurrently with continuing stress. Nicotine treatment reversed established stress-induced impairment of LTP. However, acute nicotine treatment of rats (a single dose of 1 mg/kg; sc.) did not reverse chronic-stress-induced impairment of LTP. The results show that the impairment of LTP during chronic stress can be blocked by chronic, but not acute, nicotine treatment.

Chronic psychosocial stress-induced impairment of hippocampal LTP: possible role of BDNF

Electrophysiological recording reveals that chronic nicotine treatment prevents stress-induced impairment of long-term potentiation (LTP) in the CA1 region of the hippocampus of anesthetized rats. We investigated the molecular mechanism of this action of nicotine in the CA1 region. Immunoblot analysis showed that chronic nicotine treatment (1 mg/kg, 2 times/day) normalized the stress-induced decrease in the basal levels of BDNF, CaMKII (total and phosphorylated; P-CaMKII), and calmodulin. Additionally, nicotine reversed the stress-induced increase in calcineurin basal levels. Chronic nicotine treatment also markedly increased the basal levels of BDNF in naïve rats. Furthermore, high-frequency stimulation (HFS), which increased the levels of P-CaMKII in control as well as nicotine-treated stressed rats, failed to increase P-CaMKII levels in untreated stressed rats. Compared to unstimulated control, the levels of both total CaMKII and calcineurin were increased after HFS in all groups including the stressed, but no changes were detected after HFS in the levels of BDNF and calmodulin. These results indicate that normalization by nicotine of the stress-induced changes in the levels of signaling molecules including BDNF may contribute to the recovery of LTP.